Blog for the RepRap project at www.reprap.org - a project to create an open-source self-copying 3D printer. To get all the early posts on this blog with all the images as a single PDF visit this page.

Friday, January 06, 2006

Pulley accuracy test rig

Here's the test rig I'm using to see how accurate a pulley transmission will be. The string is para-cord ø 2mm (nylon?). After putting this on the force tester, strain and creep factors were surprisingly high. Although it will just about do for the time being, I'll need better performance for Mk 2. Am thinking insulated electrical wire is a useful resource here?

I looked and looked for Kevlar string, because of it's low stretch characteristics. All of the Kevlar string I found was braided. Even though the Kevlar strands don't stretch, the braid tightens up, and the overall string does stretch. Someone on the ATM list suggested that I try an archery shop. I found a string with the brand name of Spectra. It has separate strands that are not braided, with a cloth outer jacket, and it has very little stretch. This string works well as is, but I was able to take the core (the fiberglass) out of the jacket, double it, and put it back in the jacket, making it have half as much stretch.

Hello everybody (first post on this blog)!Ed, I have a very old pen plotter with a design which reminds of your rig. It uses thin metal wires to do the job. The wires looks like bicycle-brake wires, but thinner. I think that ordinary bicycle wires could be used with good result if the radius on the 'wheels' where made big enough to accommodate for the bending radius of the wire.

This problem of stopping cables from stretching is the chief reason I try to avoid such mechanisms as much as possible - when I do use them, I personally prefer steel cable.

Another, more long-term problem is that relying on specialist cables, such as high-end bowstrings, decreases the ease with which the machine can be constructed and distributed - I doubt there are many specialist archery shops in some third world countries where the reprap technology might eventually spread. Linkages like the peaucellier or sarrus have the advantage that they could be entirely made by another fabricator using base materials, though I suppose one could use a fabricator to manufacture a specialist string-making machine and then manufacture the bowstring using that....

That's why I'm seriously exploring the option of using a Stewart platform. The control is more complex, and it takes more motors, but they're very rigid for a given amount of structure. And you could switch between bringing one tool or another to bear simply by tilting the head.

Oh, and keep looking at the Stewart platform, and also "Crank" variants which require fewer multi-directional joints. I reckoned they were vey nice but waaay to complicated for the initial attempts. Also quite expensive - 6 stepper motors...